Maybe type

[John J. Camilleri]

Hi all

While writing a resource grammar, I've found it's quite common to have possibly non-existent forms. In English for example, some adjectives have a morphological comparative ("happy" -> "happier") while others do not ("expensive" -> "more expensive"). The normal way to encode this is as a string combined with a boolean, so that if the boolean is false then we ignore the string.

But I realised this is the same as the Maybe type in Haskell, so I wrote a GF resource module to model this type. The function names and type signatures are made to mirror the Haskell implementation. Maybe is a higher-order generic type, and MaybeS is Maybe parametrised with the Str type, since that is a common case. 

The Maybe.gf module is attached - below is a small example of its use. I'm surprised this hasn't been written before. Comments are welcome - if you like it, I suggest we include this module in the RGL in the Prelude folder.

Abstract

abstract Test = {

  cat

    A ; N ; Phr ;

  fun

    caviar_N, pizza_N : N ;

    expensive_A, good_A : A ;

    more : N -> A -> N -> Phr ;

}

Concrete

concrete TestEng of Test = open Prelude, Maybe in {

  lincat

    A = {

      posit : Str ;

      comp : MaybeS

      };

    N   = SS ;

    Phr = SS ;

  lin

    caviar_N = ss "caviar" ;

    pizza_N = ss "pizza" ;

    expensive_A = {

      posit = "expensive" ;

      comp = NothingS ;

      } ;

    good_A = {

      posit = "good" ;

      comp = JustS "better" ;

      } ;

    more = \n1,a,n2 ->

      let

        cmp : Str = fromMaybeS ("more" ++ a.posit) a.comp

      in

      ss (n1.s ++ "is" ++ cmp ++ "than" ++ n2.s) ;

}

Testing on shell

Test> l more caviar_N good_A  pizza_N

caviar is better than pizza

Test> l more caviar_N expensive_A  pizza_N

caviar is more expensive than pizza